In building construction, including residential, commercial and industrial, an archway or window opening normally has a horizontally extending top structural member called a "header", and vertically extending members framing the opening called "jacks" or "legs." The arch is then often paneled with drywall, with edge beading applied to the edges of the arch opening. Where the jacks and headers are in perfect alignment, the edging provides for a perfect corner finish.
Generally, however, the headers and jacks are somewhat twisted or offset with respect to one another to some degree during installation. When this happens, the header and jacks are not square with respect to one another and are not in the same face plane on the wall side, so that when the wall around the archway is paneled with drywall, an irregularity, or imperfection, occurs at the corner of the archway. To remedy this imperfection, a relatively thick coating of drywall compound or plaster is applied to fill in the imperfection. Cracks often subsequently develop in the plaster at the corner, either from movement of the jacks and headers with respect to one another from, for instance, shrinkage or from drying out of the plaster.